1. Field of the Invention
The present invention relates to a semiconductor substrate having an SOI (Silicon On Insulator) structure, and more particularly, to a technique for gettering impurity metal contained in a semiconductor layer (SOI layer).
2. Description of the Related Art
Gettering has hitherto been performed for rendering inactive impurity metal harmful to the characteristic of a device. An SOI (Silicon On Insulator) substrate employed as a semiconductor substrate for use in a semiconductor device has a structure where a semiconductor layer (SOI layer) is provided on a support substrate made of silicon (Si) by way of a silicon oxide film (an SiO2 film) acting as an embedded insulating film. For this reason, when an attempt is made to capture the impurity metal got in the SOI layer in a gettering site on the support substrate side, several types of impurity metal, such as copper (Cu) or nickel (Ni), pass through the SiO2 film in a dispersing manner. However, many other types of impurity groups hardly pass through the SiO2 film. The impurity metal that hardly passes through the SiO2 film remains in the SOI layer or in the neighborhood of a boundary surface between the SOI layer and the SiO2 film, thereby inducing leakage failures or the like.
In relation to the group of impurity metals which hardly pass through the SiO2 film, there has been put forth a technique for forming a gettering site in the neighborhood of the boundary surface between the SOI layer and the SiO2 layer to thus capture the impurity metal on the SOI layer side. For instance, mechanical damage has been inflicted in advance on a bottom surface of the SOI layer which becomes a boundary surface between the SiO2 film and the SOI layer, thereby forming a gettering site (see, e.g., Japanese Patent Unexamined Publication no. JP-A-2002-134721). Further, there is a technique for forming a single layer region in which a plurality of minute cavities are formed along the boundary surface between the SOI layer and the SiO2 film, to thereby capture in this single layer region impurity metal, such as iron (Fe), which spreads slowly in the SiO2 film (see, e.g., Japanese Patent Unexamined Publication no. JP-A-2002-359247).
However, in relation to the SOI substrate whose film thickness is greatly reduced, when the impurity metal is captured at the boundary surface between the SOI layer and the SiO2 film, the impurity metal captured by the boundary surface exerts influence on the neighborhood of the surface of a semiconductor layer (SOI layer) which is to become an active layer of a semiconductor device, because the semiconductor layer is thin. This may cause a drop in manufacturing yield. Specifically, effective capture of impurity metal in the SOI substrate which does not affect the surface (active layer) of the semiconductor layer as in the case of a bulk substrate having a thickness of about 300 μm to 1 mm cannot be achieved.